Electrical Engineering ⇒ Topic : Behaviour of Capacitor in a D.C. Circuit

Behaviour of Capacitor in a D.C. Circuit

When d.c. voltage is applied to an uncharged capacitor, there is transfer of electrons from one plate (connected to +ve terminal of source) to the other plate (connected to -ve terminal of source). This is called charging current because the capacitor is being charged. The capacitor is quickly charged to the applied voltage and charging current becomes zero. Under this condition, the capacitor is said to be fully charged. When a wire is connected across the charged capacitor,the excess electrons on the negative plate move through connecting wire to the positive plate. The energy stored in the capacitor is dissipated in the resistance of the wire. The charge is neutralised when the number of free electrons on both plates are again equal. At this time, the voltage across the capacitor is zero and the capacitor is fully discharged. The behaviour of a capacitor in a d.c. circuit is summed up below :

(1) When d.c. voltage is applied to an uncharged capacitor, the capacitor is quickly (not instantaneously) charged to the applied voltage.

(2) When the capacitor is fully charged, capacitor voltage becomes constant and is equal to the applied voltage. Therefore, dv/dt = 0 and so is the charging current. Note that dv/dt is the slope of v-t graph of a capacitor.

(3) A capacitor can have voltage across it even when there is no current flowing.

(4) The voltage across a capacitor (Q = CV) is proportional to charge and not the current.

(5) There is no current through the dielectric of the capacitor during charging or discharging because the dielectric is an insulating material. There is merely transfer of electrons from one plate to the other through the connecting wires.

(6) When the capacitor is fully charged, there is no circuit current. Therefore, a fully charged capacitor appears as an open to d.c.

(7) An uncharged capacitor is equivalent to a *short circuit as far as d.c. voltage is concerned. Therefore, a capacitor must be charged or discharged by connecting a resistance in series with it to limit the charging or discharging current.

(8) When the circuit containing capacitor is disconnected from the supply, the capacitor remains charged for a long period. If the capacitor is charged to a high value, it can be dangerous to someone working on the circuit.